Biogas Upgrading And Energy Recovery In The Anaerobic Osmosis Membrane Reactors Enhanced By Bioelectrochemical Systems

Water scarcity and energy crisis are huge challenges for sustainable development of human being.In order to improve energy and environmental issues,government of China has formulated a plan for "peaking carbon dioxide emissions" before 2030.Therefore,development of novel wastewater treatment technology for purpose of energy recovery shows particular environmental perspective.Based on uncoupling of hydraulic retention time and sludge age,anaerobic membrane bioreactors(AnMBR),which integrate membrane separation technology and anaerobic digestion,is regarded as an important approach to enhance methane production rate.However,due to drawbacks of dissolved methane permeation through traditional membrane and low methane content in the biogas,energy input is required for secondary dissolved methane recovery and biogas upgrading,which limits energy recovery efficiency of AnMBR.In the light of above problems,a novel anaerobic forward osmosis-electrochemical membrane bioreactor(AnOMBR-MEC)was proposed to realize dissolved methane recovery,biogas upgrading and energy profit during wastewater treatment process.On one hand,dissolved methane in the reactor was in situ rejected by forward osmosis membrane.On the other hand,carbon dioxide removal and biogas upgrading were driven by bioelectrochemical system(BES)integrated with AnOMBR,which could enhance the energy recovery performance of system.The specific research contents are as follows:In this study,dissolved methane(D-CH4)rejection by forward osmosis(FO)membranes were systematically investigate to mitigate D-CH4 and energy loss.The effects of membrane materials,orientations and alginate fouling on dissolved CH4,H2 and CO2 rejections were evaluated.FO progress was further connected with Up-flow anaerobic sludge blanket(FOUASB)to recover D-CH4 from effluent.Results showed that D-CH4 rejections by CTA-ES and CTA-NW FO membranes were above 99.9%in the active layer facing feed solution(AL-FS)and active layer facing draw solution(AL-DS)orientations.Dissolved H2 rejections achieved higher than dissolved CO2 rejections in AL-FS orientation which was attributed to difference of dissolved gas permeability.Alginate fouling layer improved dissolved H2 rejection while concentrative internal concentration polarization aggravated dissolved CO2 rejection by fouled CTA-ES membrane in AL-DS orientation.D-CH4 was successfully collected by FO-UASB due to FO rejection and liquid-to-gas mass transfer.The maximum total D-CH4 collection rate was 57.9 and 77.6 mg COD/(L·d)at 35 ? and 25?,respectively.In order to overcome low biomethane content of AnOMBR,a novel anaerobic osmotic membrane bioreactor-microbial electrolysis cell(AnOMBR-MEC)system was developed for upgrading biogas and wastewater treatment based on cathodic reduction of CO2 and anaerobic digestion simultaneously.The AnOMBR-MEC elicited an 95%of soluble chemical oxygen demand(sCOD)removal and 100%of phosphorus removal.As the experiment progressed,unwanted CO2 produced from biogas was reduced to formate using a SnO2 cathode in the electrocatalytic-assisted MEC,with the highest faradic efficiency of formate being 85%at 1.2 V.Microbial community analysis revealed that hydrogenotrophic methanogens converted produced H2 and formate to methane when formate-enriched catholyte was pumped into feed tank.Compared to AnOMBR,methane content increased from 55%to 90%at the end of operation and methane yield experienced a 1.6-fold increment in the AnOMBR-MEC system.In order to further improve energy recovery efficiency of AnOMBR,performance of AnOMBR under draw solution(DS)concentrations and temperature were investigated.The results showed that the highest water production capacity of AnOMBR were achieved under DS concentration of 0.75 M.When DS concentration exceeded this critical value,aggravated membrane fouling and cake-enhanced concentration polarization caused a negative contribution for net driving force maintenance of FO progress.The specific methanogenic activity of acetoclastic methanogens was suppressed due to rapid reverse solute diffusion.Under the critical DS concentration,removal of sCOD was above 90%during 60 days operation of AnOMBR.D-CH4 in the bulk solution were not oversaturated at a ratio of 0.66,0.48 and 0.17 and energy recovery of 2.0,3.1 and 4.2 kWh/m3 was achieved at 15?,25? and 35?,respectively.The energy recovery of AnOMBR-MEC system was achieved at 5.1 kWh/m3 and corresponding net energy profit of system reached at 1.57 kWh/m3 at 25?.Finally,enhanced net energy profit in the AnOMBR-MEC system were further achived by alkalinity absorption and electromethanogenesis.Compared to open control reactor,stainless steel-carbon felt composite biocathode system(BES-SSCF)showed higher methane production rate,and hydrogenotrophic methane production and prones consumption were strengthened.Methane composition in the biogas,methane yield and methane production were 92%and 0.31 L/gCODremoval,respectively which increased by 53%and 55%,respectively.When the applied voltage was 1.0 V,a large number of agglomerated viable bacteria were enriched on cathodic biofilm,and the thickness of cathode biofilm reached 175 ?m.The AnOMBR-MEC system equipped with BES-SSCF biocathode was developed for energy recovery.The pH value increased under dual effects of electromethanogenesis and alkalinity set-up,which promoted transform of carbonous matters from carbon dioxide to bicarbonate through alkalinity absorption.Bicarbonate could be further converted into methane by BES-SSCF biocathode under applied voltage of 1.0 V,which enhanced methane content and methane production performance of reactor and net energy recovery efficiency reached 2.54 kWh/m3.The results demonstrated that AnOMBR-MEC could achieve biogas upgrading and net energy recovery.